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 <h2 class="title"><span class="function"><strong>require_once</strong></span></h2>
 <p class="verinfo">(PHP 4, PHP 5)</p>
 <p class="para">
  <em>require_once</em> 语句和 <span class="function"><a href="function.require.html" class="function">require</a></span>
  语句完全相同，唯一区别是 PHP 会检查该文件是否已经被包含过，如果是则不会再次包含。
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 <p class="para">
  参见 <span class="function"><a href="function.include-once.html" class="function">include_once</a></span> 的文档来理解
  <em>_once</em> 的含义，并理解与没有 <em>_once</em> 时候有什么不同。
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